The present invention relates to an ignition system for an automobile and, particularly to an ignition system for converting the direct current for igniting an engine.
An example of a known ignition system for an automobile is shown in FIG. 3 which includes a battery 1 for supplying power which is converted into a high voltage signal by a DC-DC converter 2. This converted voltage signal is supplied to an ignition condenser or capacitor 4 through a discharging circuit 3 including a thyristor 3a. An LC resonance circuit is formed by a condenser 2a, a choke coil 3b and the condenser 4. The LC resonance circuit discharges a charged capacitance of the condenser 2a to the condenser 4 which is charged with almost twice the voltage of the DC-DC converter output voltage. The charged energy of the condenser 4 corresponds to an ignition energy for one spark. An engine computer 5 supplies a pulse signal which indicates the ignition duration in accordance with a throttle, engine revolution, etc. An ignition control circuit 6 consists of a control circuit 6a, an oscillator 6b and a switching transistor 6c. The control circuit 6a supplies an ignition signal SA in accordance with the pulse signal of the engine computer 5. The oscillator 6b oscillates in a certain period while the ignition signal SA is applied. This oscillator 6b consists of a mono-stable multi-vibrator and serves to determine the charge and discharge time of the condenser 4 in accordance with a charge signal SC and a discharge signal SB, respectively. The switching transistor 6c turns ON when the discharge signal SB is applied and allows the discharged current to flow from the condenser 4 to the choke coil 7 and a transformer 8. Another LC resonance circuit is formed by the condenser 4, the choke coil 7 and the transformer 8. A discharged current increases along with the oscillation period of the LC resonance circuit and becomes a maximum voltage when the discharge of the condenser 4 is completed. The primary current of the transformer 8 is connected to ground. The second current of the transformer 8 is connected to a spark plug 10 so that the magnetic energy of the transformer at the second current converts into a spark. The known circuit includes a clamp circuit 11 for setting the voltage applied to the switching transistor 6c to a predetermined voltage level.
In the known ignition system, the oscillator 6b is a self oscillator and determines the charge and discharge time of the condenser 4. Consequently, the ignition system is not able to change the discharge durations of the spark plug 10 in order to stabilize the engine combustion.